1. Field of the Invention
The subject invention relates to multi-axis controllers and, more particularly, controllers that provide output signals in proportion to applied command forces.
2. Description of the Prior Art
Multi-axis controllers, such as those used to control aircraft flight control surfaces, have had a long history of development. Early aircraft control systems employed relatively complex hand and foot operated devices to control the flight surfaces. To simplify the flight controls, control systems were developed in which a single controller generated control signals in response to operator commands.
One of the earlier types of such controllers came to be known as a "position controller". Position controllers employed a variety of electrical and mechanical arrangements for coordinating the physical displacement of a single control handle with the amplitude strength and direction of control signals. Examples are shown in U.S. Pats. Nos. 3,011,739; 3,028,126; 3,056,867; 3,580,636; 3,771,037; 3,881,106; 4,012,014; and 4,069,720. Other prior art position controllers such as shown in U.S. Pats. Nos. 2,811,1047; 3,814,199; 3,886,361; and 4,250,378 attempted various schemes for utilizing optical signals to generate control signals in response to operator commands. Basically, these position controllers were light-modulating analog devices wherein the controller modulated light from a given source to provide control signals.
One problem with position controllers known in the prior art stemed from their requirement for relatively large physical displacements of the controller handle. In a dynamic flight path, pilots typically experience disorientation due to rate and angular accelerations. This made the position controller difficult and unnatural to operate. Effective operation of the position controller demanded a high degree of coordination from the pilot and required substantial training and familiarization. Even then, under conditions of high rate and angular acceleration, the position controller was difficult to accurately and reliably manipulate.
It has been found that controllers having relatively low mechanical displacement but requiring relatively high manipulating forces were more natural for pilots to operate. Such controllers, sometimes referred to as "force controllers", provided improved control--particularly under high rate and fight path accelerations. Indeed, it was found that even with less familiarization, pilots could generally operate force controllers with greater repeatability and reliability than position controllers. Examples of force controllers are shown in U.S. Pats. Nos. 3,149,806; 3,304,799; 3,454,920; 3,523,665; and 3,729,990. Among force controllers known in the prior art were the type wherein a flexible tube generated command signals in response to lateral and torsional forces applied to the controller handle. Examples of this type of controller are shown in U.S. Pats. Nos. 2,895,086; 3,167,667; and 3,707,093.
While prior art force controllers were better suited to the needs of pilots than position controllers, the force controllers were generally limited in that they provided output signals in response to force commands in only three control axes. In some applications, such as helicopters, there was a need for a force controller that provided output signals in response to force commands in a fourth control axis.
Prior art force controllers that have included electrical position sensors such as potentiometers, inductive sensors, and piezoelectric devices have had other disadvantages. A persistent difficulty with such controllers has been accurately and reliably translating force commands into output control signals.
Another disadvantage of many prior art force controllers has been that they are analog devices that are not directly compatible with present flight control systems. For many years, flight control systems were basically mechanical systems and the controllers that were developed and used for these systems were basically analog devices. However, as flight control systems developed, digital electrical control systems supplanted the mechanical systems. Even after electrical flight control systems gained acceptance, the analog type controllers continued to be used because they had proven to be reliable and inexpensive. Usage of the analog controllers with the digital flight control systems required additional analog-to-digital converter hardware. Moreover, the analog controller introduced certain processing errors that were inherent to an analog signal.
Digital force controllers have been used in some prior art applications to avoid the additional analog-to-digital converter hardware required by the analog controllers as well as the inherent analog signal processing errors. Thus, they are generally more reliable and less expensive than analog force controllers. However, these digital force controllers have generally been designed for an "absolute" digital system wherein quantized changes in signal parameter are compared to a constant reference. As digital control systems continue to develop, "incremental" digital systems have become preferable to absolute digital systems. In "incremental" digital systems the net change in a quantized signal is maintained on a continuous basis. Incremental digital systems have the same advantages over analog systems as the absolute digital systems, but are still more reliable and less expensive to build and maintain than absolute digital systems.
Accordingly, there was a need in the prior art for a force controller that would provide control signals in response to pilot commands applied to four control axes. Preferably, the force controller would be digital and would directly interface with an incremental digital control system. In particular, an optical type incremental digital force controller would be preferred because it would be immune from electromagnetic and electrical interference.